Mixing and dispensing apparatus

ABSTRACT

A mixing and dispensing apparatus comprises at least one horizontal main cylinder having a row of inlet openings at its top and a row of outlet openings at its bottom. Two pistons are displaceable in the cylinder in such manner that they may either be independently displaced, or simultaneously displaced at a constant distance from each other. At the end of each displacement, only one opening is uncovered. For preparing a dilution series, a separate inlet opening is provided for the liquid to be diluted and the other inlet openings are connected with a feed line for the diluent.

[ 1 June 13, 1972 3,285,669 11/1966 Primary Examiner-Samuel F. Coleman Assistant Examiner-John P. Shannon, Jr. Attorney-Waters, Roditi, Schwartz & Nissen 57 ABSTRACT A mixing and dispensing apparatus comprises at least one horizontal main cylinder having a row of inlet openings at its top and a row of outlet openings at its bottom. Two pistons are displaceable in the cylinder in such manner that they may either be independently displaced, or simultaneously displaced at a constant distance from each other. At the end of each displacement, only one opening is uncovered. For preparing a dilution series, a separate inlet opening is provided for the liquid to be diluted and the other inlet openings are connected with a feed line for the diluent.

7 149 m li sPtw nslisur Jan Waldman, l3 Raadhuisplein, Zandvoort, Netherlands Dec. 9, 1970 Foreign Application Priority Data Dec. 10, 1969 Netherlands..........................6918495 .222/129, 222/217, 222/330, 222/361, 23/259, 195/1035 R, 195/127 .B67d 5/56 .222/216, 217, 129, 330, 305, 222/307, 361; 23/253 R, 259

References Cited UNITED STATES PATENTS 3,221,948 12/1965 Kalist.................................222l36l X United States Patent Waldman [54] MIXING AND DISPENSING APPARATUS [72] Inventor:

[22] Filed:

[21] Appl.No.: 96,360

[58] FieldoiSearch.................

Fit" PATEMIEDJUH 13 1572 saw 3 0F 4 fig.4

MIXING AND DISPENSING APPARATUS BACKGROUND OF THE INVENTION The invention relates to an apparatus for mixing liquids and for dispensing at least one mixture in a predetermined dose, and in particular to an apparatus of this kind for producing a series of consecutive dilutions of a given liquid.

The expression liquid" is intended hereinafter to cover both pure liquids and suspensions.

The known mixing and dispensing arrangements are generally complex and bulky and include valve systems of such kind that they are difiicult to handle and to clean and that they can not be satisfactorily sterilized.

Moreover, the known arrangements are unsuitable for preparing a series of dilutions as necessary for serological reactions and the like. Up to now all the labor involved in the preparation of such a series had to be manually performed by laboratory workers. It will be clear that the use of a mechanical apparatus for this purpose would provide large savings of labor. In addition, such an apparatus might increase the value of the tests since the apparatus would always show the same deviations, if any, from the proper dilution values, whereas such deviations vary in random fashion if the series is prepared by hand. Thus, the apparatus would give a more exact picture in cases wherein two samples must be compared with each other. For practical reasons, it is usual in serological reactions to add the serum to the diluent although this may sometimes give rise to hydrolysis of the antibodies. A mechanical apparatus might provide the possibility to add the diluent to the serum whereby any hydrolysis would be avoided.

It is an object of the invention to provide a simple mixing and dispensing apparatus which is easy to clean and may be sterilized as a whole.

It is a further object of the invention to provide an apparatus of this kind which may be used for preparing a series of consecutive dilutions of a given liquid. I

SUMMARY OF THE INVENTION The apparatus according to the invention comprises at least one main cylinder provided with a plurality of inlet openings for liquids and with at least one outlet opening for a mixture, two pistons sealingly enclosed by said main cylinder, and a driving mechanism adapted to displace either of said pistons independently and to displace both pistons simultaneously at a constant distance from each other, in such manner that at most one of said openings is uncovered at the end of each displacement.

In a preferred embodiment of the invention, the main cylinder is formed as a horizontal tube having a row of longitudinally spaced inlet openings at its top and a row of correspondingly spaced outlet openings at its bottom, the outlet openings being staggered with respect to the inlet opening. The apparatus may comprise a plurality of main cylinders arranged in parallel and means for interconnecting corresponding pistons in all main cylinders.

It the apparatus is used to produce a series of consecutive dilutions of a given liquid the inlet openings and the outlet openings in the said rows are equally spaced and the inlet openings of said row are connected with a feed line for the diluent, while a separate inlet is provided for the liquid to be diluted. This separate inlet may be arranged at different locations according to the reaction which is to be carried out with the dilution series. For a complement binding reaction, such as the Wassermann test, the undiluted serum is used as a control liquid andin' this case, the inlet for the liquid to be diluted precedes the first inlet opening for the diluent at a distance corresponding with the distance between the inlet openings for the diluent. For an agglutination reaction, however, the diluent is used as a control liquid and in this case, the separate inlet opening for the liquid to be diluted is placed between the first and the second inlet opening for the diluent.

It is preferred to apply nipples to the inlet and outlet openings which seal the openings and do not protrude into the interior of the main cylinder. Preferably, the cavity of the inlet nipples for the diluent is tapered in such manner that the opening discharging into the main cylinder is at the narrow end, whereas the cavity of the outlet nipples is substantially cylindrical. The inner diameter of the inlet nipple for the liquid to be diluted should be of such size as to allow for the passage of the capillary tube of a Pasteur pipette. The inner diameter and the wall thickness of all outlet nipples are preferably the same.

For sterilization purposes it is important that the pistons may be temporarily placed in a somewhat widened part of the main cylinders and to this end the main cylinder is preferably provided with flaring portions at both ends. The main cylinder is preferably made of glass. This pistons, too, may be made of glass.

The driving mechanism, which actuates thepistons in the main cylinder, should allow for a disconnection of the pistons,

if so desired. When reconnecting the pistons they should be in fixed starting positions. The driving mechanism should be adapted to move the pistons according to a specific schedule, which should be interchangeable in order to establish another dilution program.

The invention is now further elucidated with the accompanying drawing.

SHORT DESCRIPTION OF THE DRAWINGS FIG. 1 shows an elevation of a preferred embodiment of the invention, by means of which a dilution series of one-half, onequarter and one-fourth on, may be produced.

FIG. 2 shows a top view of this same apparatus on a somewhat smaller scale, in which details have been left out.

FIG. 3 shows on an enlarged scale a longitudinal cross-section of the main cylinder, in which only the portion comprising the mixing chamber has been represented.

FIG. 4 shows in outline the successive positions of the piston reference to rods in the main cylinder during the formation of the various dilutions for the case that the control sample consists of the liquid to be diluted.

FIG. 5 shows a corresponding diagram for the case that the control sample consists of the diluent.

DESCRI T ON OF A PREFERRED EMBODIMENT In the drawing the main cylinder is indicated by reference number 1; it is provided with flaring end portions 2, for purposes to be further elucidated hereinbelow. In the main cylinder 1, the pistons 3 and 4 are located, which have been shown in FIG. 1 as solid rods. The free ends of the pistons 3 and 4 are attached to the drive straps, 5 and 6 respectively. For the sake of convenience, the drive strap '6 is shown in shortened fashion; it extends far to the outside in reality, because the piston 4 is almost in its outermost position. The piston 3, however, is almost completely pushed-in.

At the top of the main cylinder 1, inlets have been provided, in which inlet nipples 7 have been fixed, which are in turn connected with tee-pieces 8, inserted in a feed line 9 for the diluent. This feed line is connected with a supply vessel for the diluent (not shown).

In the bottom of the main cylinder outlets have been provided, in which outlet nipples 10 have been fixed. Receptacles 11-22 are placed below each outlet nipple l0 and serve to receive the. control sample (vessel 11) and the series of dilutions (vessels 12-22). In a separate position at the top of the main cylinder, just past the first outlet (that is'to the right thereof in the drawing), an inlet nipple 23 for the liquid to be diluted is provided. The opening of this nipple should be so wide that the capillary tube of a Pasteur pipette may pass through this nipple and reach the bottom of the main cylinder, if piston 3 uncovers its opening. In the drawing, this inlet nipple is shown as a funnel and this is indeed a convenient embodiment thereof, because it allows to use this nipple for receiving the diluent, too, for instance during rinsing. The feed line 9 is provided with a curved end portion for this purpose,

which can be placed above the funnel-shaped end of the inlet nipple 23 for the liquid to be diluted, if so desired.

Advantageously, the apparatus may also be used for administration of the reagents to the dilution vessels. In that case the reagents are not diluted, so that the feed line for the diluent may be disconnected from the supply vessel for the diluent and connected, for instance, with the inlet nipple for the liquid to be diluted. This inlet nipple is then used for receiving and distributing the reagents.

The main cylinder 1 is incorporated in a frame 24 with a base 25, in which the driving mechanism of the apparatus is accommodated (not shown). Shown in the drawing are only the conveying gear-wheels 26 and 27, which may be induced in a known manner to mesh with gear racks 28 and 29, respectively, provided at the bottom side of the drive straps 6 and 5. The actuation of the piston may also be effected by any other disengageable means, for instance, by belts or friction rollers.

FIG. 2 shows the machine from above, omitting details of the liquid inlets and outlets for claritys sake. In order to get full benefit of the apparatus, it is advantageous to mount as many main cylinders in parallel as practicable and to connect the piston of these cylinder on each side with a common drive strap.

FIG. 3 serves to explain the actual mixing process and shows an enlarged longitudinal section of the main cylinder, representing only that part thereof in which the mixing chamber is situated at that moment. The pistons are shown in the positions in which the mixing chamber has just been filled with liquid coming from the inlet nipple 7; this liquid has obtained a higher speed by virtue of the tapered shape of the nipple 7, which promotes the mixing in the mixing chamber. If, nevertheless, an insufficient mixing should occur, which might be the case, for instance, with viscous liquids, a small stirring magnet 30, incorporated in a synthetic material such as Per- Ion may be introduced into the mixing chamber through the separate inlet nipple 23 for the liquid to be diluted, which nipple will generally have a larger inner diameter than the other inlet nipples; the stirring magnet is brought into rotation in known manner by means of a rotating magnetic field.

In order to prevent a penetration of the nipples into the interior of the main cylinder, it is advantageous to provide the nipples at the outside in the proper place with a collar 31, which prevents insertion of the nipple beyond a certain level.

FIG. 4 explains the operation of the apparatus while making a dilution series of 1:2, 1:4, 1:8 and so on, fora complement binding reaction, that is for a reaction in which the control sample consists of the undiluted serum.

In FIG. 4a, the starting position of the pistons in the main cylinder is indicated. The piston 4 is situated with its end face just in front of the inlet nipple for the liquid to be diluted, whereas piston 3 is situated with its end face just in front of the first outlet opening. The distance between both pistons is such that a space is formed having twice the volume which should be squeezed out into the control vessel and into allvdilution vessels. This distance is 2a. The space between the pistons is now filled through the separate inlet nipple 23 and by means of a Pasteur pipette (of course by hand) with the liquid to be diluted. However, the pipette needs no calibration and an adjustment of the pipette volume by flowing off surplus liquid is also unnecessary, because the apparatus itself defines the liquid volume comprised by the mixing chamber. Moreover, the pipette needs be used only once.

After introduction of so much liquid to be diluted as necessary to fill the complete mixing chamber together with part of the inlet nipple, the apparatus is switched on and the further dilution cycle, which is partially shown in FIGS. 4b-4n, is performed completely automatically.

When the apparatus is switched on, both pistons are first simultaneously shifted over a distance a in the direction of the piston 3 (to the right), while maintaining the distance 2a between them. The situation then arising is illustrated in FIG. 4b. The mixingchamber, filled with a double, volume of the liquid to be diluted, is now in communication with the first outlet nipple 10, whereas the inlet nipples 23 and 7 are closed. The liquid is prevented from flowing out through the outlet nipple by the atmospheric pressure.

Subsequently the piston 4 is shifted to the right over a distance a, keeping the position of the piston 3 unchanged. This causes one volume of the liquid to be diluted to be squeezed out through the first outlet nipple 10 into the control vessel (11 in FIG. .1). The situation now obtained is illustrated by FIG. 4c.

Now both pistons are again shifted to the right, maintaining the distance between them, which now is a. Thus the situation of FIG. 4d arises. The mixing chamber now communicates with the first inlet nipple for the diluent. Displacing piston 3 to the right over a distance a while keeping the position of piston 4 unchanged will double the volume of the mixing chamber, so that one volume of the diluent is sucked in through the inlet nipple 7. The position now obtained is illustrated in FIG. 4e.

Again both pistons are shifted in the direction of piston 3, while maintaining the distance a between them. This gives rise to the situation of FIG. 4f By displacement of the piston 4 over a distance a to the right, a volume of liquid diluted in a ratio of 1:2 is squeezed out into the first dilution vessel (12 in FIG. I), provided that the position of piston 3 remains unchanged (FIG. 4g).

The process may be repeated as many times as necessary, the transitions from FIG. 4g to FIG. 4h, from FIG. 4h to FIG. 4k, from FIG. 4k to FIG. 41 and from FIG. 41 to FIG. 4m correspond, respectively, with the transitions from FIG. 40 to FIG. 4d, from FIG. 4d to FIG. 4e, from FIG. 4e to FIG. 4f and from FIG. 4f to FIG. 4g. Any further steps are performed in an analogous manner. 7

Another embodiment of the main cylinder is represented in FIG. 5, also showing the production of a dilution series of 1:2, 1:4, 1:8 and so on, but with the control sample consisting of the diluent, as is usual, for instance, in agglutination reactions.

Only the initial phases of the process shown in FIG. 5 are different; starting from FIG. 5f the process is identical with the process according to FIG. 4 starting from FIG. 4d.

In FIG. 5a, the piston are in starting positions suitable for the production of the control sample, which now consists of the pure diluent. The pistons engage each other with their end faces just in front of the first inlet nipple 7.

In order to produce the control sample, the piston 4 is now kept in its initial position, whereas the piston 3 is shifted over such a distance that the first outlet just remains covered. The amount of liquid sucked in must be completely squeezed out into the control vessel, because the apparatus should be empty again for reception of the liquid to be diluted. Thus piston 3 must be shifted over a distance a. The positions now reached are represented by FIG. 5b. Since the space between the pistons isin communication with the first inlet nipple 7 for the diluent, this space is automatically filled with the diluent during the displacement of the piston 3.

Subsequently, both pistons are moved in the main cylinder over a distance d while maintaining their mutual distance a, d

, representing the diameter of the inlets and outlets. Simultaneously, an inlet will be closed and an outlet will be opened by this action. Now the situation is as shown in FIG. 5c. Thereafter, the piston 3 is maintained in its position and the piston4 is displaced to the right over a distance a, so that the end faces of the pistons are brought into engagement again. Thus, all liquid present in the mixing chamber is squeezed out through the outlet 10. The situation then corresponds to FIG. 4d.

After going through phases 5a-5d, the control sample, now consisting of the diluent is available in vessel 11 (FIG. 1). Piston 3 is now shifted over a distance a to the right, so as to form a mixing chamber accessible through the separate inlet nipple 23, which is arranged, according to FIG. 5, between the first and the second inlet nipple for the diluent. Through nipple 23, an amount of the liquid to be diluted may now be introduced with a Pasteur pipette. The further operations are completely analogous to the operations according to FIG. 4, with FIGS. 4d-4ncorresponding to FIGS. 5f-5p.

The above-described operational cyclemay be repeated for a substantial number of times. The required displacements of the pistons are of a simple nature and can be readily realized, for instance, by means of a mechanism with interchangeable cam disks.

in this respect, it is observed that the proper concentration series is actually obtained in the above-described manner only when all outlet nipples are freed from liquid. This is realized by closing the feed line 9 after deaeration with extended pistons, and thereafter moving the pistons towards each other until their end faces meet. The air in the main cylinder can only escape through the outlets whereby any remaining liquid is blown out. In serological reactions, this air need not to be sterile.

After dilution of the liquid to be diluted (the serum), one or more reagents are added to the serum dilutions in all serological reactions. To each serum dilution, an amount of reagent is added which is equal to the volume of the serum dilution itself. The same holds true for a second and a third reagent. For the addition of the reagents, the same apparatus may be used. After rinsing for removing all serum residues, the feed line 9 is connected with the inlet nipple 23 and the reagent is fed through the feed line. The apparatus may operate according to the same program as before to deliver the same volume of reagent to each receptable. Moreover, the volume of the administered reagent is equal to the volume of the serum dilution already present in the receptacle.

Before administration of a subsequent reagent, the rinsing and deaerating should be repeated, of course. During this rinsing and deaerating the receptacles should not be present, so that they should be easily removable.

It is observed that in serological reactions the fact that the amounts of the serum dilutions and the amounts of the added reagents are equal, is much more important than the absolute values they might have. Thus, the fact that a certain amount of liquid remains in the outlet nipples does not interfere at all, because this amount is the same under all conditions.

Other dilution programs than described hereinbefore may be easily realized by the substitution of a difierent main cylinder with modified distance between the openings and of other cam disks or similar interchangeable programming devices.

Before using the apparatus according to the invention, the main cylinders with their pistons, nipples, feed lines, and so on, should be thoroughly cleaned, of course, and for applications in the bacterological field, they should also be sterilized. To this end it is advantageous that the main cylinders comprise the flaring end portions indicated by reference number 2 in FIG. 1. During sterilization both pistons are extracted so far from the main cylinders that their end faces lie within the flaring end portions. Thus, a possible irregular expansion of the pistons or of the main cylinder cannot lead to breakage.

Furthermore, it is advantageous for sterilizing when the main cylinders with pistons and further accessories are mounted in a detachable frame, which may be easily removed from the driving mechanism. It will then suffice to detach this frame, after which it can be sterilized in its entirety. After sterilization, the inlet nipples should be all filled with the diluent and deaerated. This may be done with the pistons in the extended as well as in the pushed-in positions. Deaeration with pushed-in pistons (for example, by knocking) provides the advantage that the outlet nipples stay free from liquid.

In view of the sterilization of the apparatus, the materials from which the different parts are made should be chosen carefully. Therefore, the main cylinders and the pistons are preferably made from glass, whereas the nipples, connections and so on are mainly made from a heat resisting synthetic resin, which may withstand at least a temperature of 120 Centigrade.

After sterilization and deaeration, the detached frame is mounted again on the driving mechanism and the receptacles are placed on the bed plate positioned below the main cylinders. These receptacles may be reaction tubes, placed in a specially designed rack, or in holes of a plastic plate fitting to the bed plate.

In order to avoid contamination of the dilutions, it is desirable to press the bed plate upwards as far as possible after insertion of the receptacles, so that the bed plate should be movable, preferably in a vertical direction.

With respect to the narrow inlets and outlets, care should be taken to perform the displacements of the pistons whereby liquid is sucked in or squeezed out not too fast, in order to avoid leakage past the pistons. The transportation movements may be performed at high speed.

The apparatus depicted hereinabove is especially advantageous in performing serological reactions, bacterial sensitivity determinations, detenninations of antibiotic blood levels and determinations of bacteria, in which a series of culture media are filled and innoculated. All serological reactions in which the concentration (titre) of antibodies should be detemiined in a patients serum may be performed with the apparatus according to the invention. Mention may be made of the Wassermann reaction, the Waller-Rose test and Widal's agglutination reactions. in these serological reactions, which are to be performed very frequently, itwas up till now impracticable to carry out all reactions in a large series of dilutions. Mostly only two or three dilutions were tested. Thereafter the positive sera were titrated out in a complete dilution series in order to determine the concentration (titre) of the antibodies. With the apparatus according to the invention each determination may be immediately performed in the complete dilution series, because this will take almost no additional time. Then the proper titre is immediately known, too.

The determination of the antibiotics content of the blood (the blood level) according to the Rammelkamp method is another example of a method which involves so much labor, that it is only performed out of extreme necessity. However, with the dilution apparatus according to the invention, it is possible to carry out the determination as a routine test, thus providing an efficient method for controlling the effect of antibiotics administration. Especially with. patients with a bad kidney function, who have difficulty in excreting the antibiotic, such a control is important.

The determination of bacteria count in urine, ascites liquid and other liquids, in which use is made of dilution series, may be carried out with the apparatus according to theinvention. In these cases, however, the receptacles are replaced by a large plate with a solid culture medium.

In the determination of bacteria, the bacterium to be determined is inoculated on a series of mutually different culture media. The apparatus according to the invention may be used for the production of the series of culture media as well as for the inoculation thereof in the following manner.

Each main cylinder of the apparatus receives through feed line 9 (and inlet nipple 23 connected therewith) one of the different culture media and dispenses a constant volume of this specific culture medium into the corresponding receptacles. This gives rise to a number of rows, of which each row consists of the same culture medium, while all rows are mutually different.

After this, the rack with receptacles is turned through an angle of thus placing the main cylindersover and above a number of identical rows of mutually different culture media.

Now the apparatus is used, in the same manner as for dispensing the culture media, for dispensing equal volumes of the bacterium strains to be determined.

It should be noticed that rather large volumes of the culture medium are often necessary for carrying out these determinations, which may lead to an adaptation of the dimensions of the apparatus, unless a micro-method is used.

It will be clear that the apparatus according to the invention has many advantages. First of all, a substantial saving of time is obtained and less man-power is needed. In the use of the pipette, no measurement errors can be made. if there is a deviation in the apparatus, this deviation will automatically always be the same. Thus a better possibility is obtained for comparing two samples of the same patient, taken at different times. The rinsing of large amounts of pipettes, which was formerly necessary, and which involved a lot of breakage,

becomes redundant. The apparatus may be easily sterilized in its entirety.

Secondly it is made possible by the apparatus according to the invention to carry out determinations in a routine manner which could not or only exceptionally be performed before. Moreover, due to the rapid operation of the apparatus, the first made dilutions need not stand for a too long time before the subsequent reagents are added. This is especially important for large numbers of tests, as necessary, for instance, for the Wassermann reaction.

Thirdly, an initial screening becomes redundant and an immediate result is obtained. The pro-zone effect is no longer troublesome.

Finally, the dilutions are produced with the apparatus according to the invention in a theoretically more correct manner than was hitherto usual in view of practical considerations. The diluent is added to the serum, thus substantially diminishing the chances of hydrolysis of the antibodies.

I claim:

1. Apparatus for mixing liquids and for dispensing at least one mixture in a predetermined dose, comprising at least one main cylinder provided with a plurality of inlet openings for liquids and at least one outlet opening for a mixture, two pistons sealingly enclosed by said main cylinder, and a drive mechanism adapted to displace either of said pistons independently and to displace both pistons simultaneously at a constant distance from each other, in such manner that at most one of said openings is uncovered at the end of each displacement.

2. Apparatus as claimed in claim 1, wherein said main cylinder is formed as a horizontal tube with a row of longitudinally spaced inlet openings at its top and a row of correspondingly spaced outlet openings at its bottom, the outlet openings being staggered with respect to the inlet openings.

3. Apparatus as claimed in claim 2 for producing a series of consecutive dilutions of a given liquid, further comprising a feed line for the diluent, wherein said inlet and outlet openings are equally spaced and said inlet openings are connected with said feed line, while a separate inlet opening is provided for the liquid to be diluted.

4. Apparatus as claimed in claim 1, wherein each independent displacement of a piston extends through the distance required to admit or to dispense, respectively, a required liquid volume, and each simultaneous displacement of both pistons extends through the distance required to render a subsequent admission or dispensing possible.

5. Apparatus as claimed in claim 4, wherein all displacements of the pistons are in the same direction, in such manner that independent displacements of one of the pistons serve to admit a liquid, and that independent displacements of the other piston serve to dispense a liquid.

6. Apparatus as claimed in claim 3, wherein said separate inlet opening for the liquid to be diluted precedes the first inlet opening of said row at a distance corresponding with the distance between subsequent inlet openings of said row.

7. Apparatus as claimed in claim 3, wherein said separate inlet opening for the liquid to be diluted is arranged between the first and the second inlet opening of said row just past the first outlet opening.

8. Apparatus as claimed in claim 3, further comprising a plurality of nipples, each applied to one of the inlet and outlet openings sealing said openings and spaced from the interior of said main cylinder.

9. Apparatus as claimed in claim 8, wherein the cavity of the inlet nipples for the diluent is tapered in such manner that the opening discharging into the main cylinder is at the narrow end, and wherein the cavity of the outlet nipples is substantially cylindrical.

10. Apparatus as claimed in claim 9, wherein all outlet nipples have the same inner diameter and the same wall thickness. 7

11. Apparatus as claimed in claim 8, wherein the inlet nipple for the liquid to be diluted is cylindrical and allows for the passage of the capill tube of a Pasteur pipette.

12. Apparatus as c aimed in claim 1, wherein said main cylinder is provided with flaring end portions at both ends.

13. Apparatus as claimed in claim 1, wherein said main cylinder and said pistons are made of glass.

14. Apparatus as claimed in claim 1, comprising a plurality of main cylinders arranged in parallel, and means for interconnecting corresponding pistons in all said main cylinders. 

1. Apparatus for mixing liquids and for dispensing at least one mixture in a predetermined dose, comprising at least one main Cylinder provided with a plurality of inlet openings for liquids and at least one outlet opening for a mixture, two pistons sealingly enclosed by said main cylinder, and a drive mechanism adapted to displace either of said pistons independently and to displace both pistons simultaneously at a constant distance from each other, in such manner that at most one of said openings is uncovered at the end of each displacement.
 2. Apparatus as claimed in claim 1, wherein said main cylinder is formed as a horizontal tube with a row of longitudinally spaced inlet openings at its top and a row of correspondingly spaced outlet openings at its bottom, the outlet openings being staggered with respect to the inlet openings.
 3. Apparatus as claimed in claim 2 for producing a series of consecutive dilutions of a given liquid, further comprising a feed line for the diluent, wherein said inlet and outlet openings are equally spaced and said inlet openings are connected with said feed line, while a separate inlet opening is provided for the liquid to be diluted.
 4. Apparatus as claimed in claim 1, wherein each independent displacement of a piston extends through the distance required to admit or to dispense, respectively, a required liquid volume, and each simultaneous displacement of both pistons extends through the distance required to render a subsequent admission or dispensing possible.
 5. Apparatus as claimed in claim 4, wherein all displacements of the pistons are in the same direction, in such manner that independent displacements of one of the pistons serve to admit a liquid, and that independent displacements of the other piston serve to dispense a liquid.
 6. Apparatus as claimed in claim 3, wherein said separate inlet opening for the liquid to be diluted precedes the first inlet opening of said row at a distance corresponding with the distance between subsequent inlet openings of said row.
 7. Apparatus as claimed in claim 3, wherein said separate inlet opening for the liquid to be diluted is arranged between the first and the second inlet opening of said row just past the first outlet opening.
 8. Apparatus as claimed in claim 3, further comprising a plurality of nipples, each applied to one of the inlet and outlet openings sealing said openings and spaced from the interior of said main cylinder.
 9. Apparatus as claimed in claim 8, wherein the cavity of the inlet nipples for the diluent is tapered in such manner that the opening discharging into the main cylinder is at the narrow end, and wherein the cavity of the outlet nipples is substantially cylindrical.
 10. Apparatus as claimed in claim 9, wherein all outlet nipples have the same inner diameter and the same wall thickness.
 11. Apparatus as claimed in claim 8, wherein the inlet nipple for the liquid to be diluted is cylindrical and allows for the passage of the capillary tube of a Pasteur pipette.
 12. Apparatus as claimed in claim 1, wherein said main cylinder is provided with flaring end portions at both ends.
 13. Apparatus as claimed in claim 1, wherein said main cylinder and said pistons are made of glass.
 14. Apparatus as claimed in claim 1, comprising a plurality of main cylinders arranged in parallel, and means for interconnecting corresponding pistons in all said main cylinders. 